1. Field of the Invention
The present invention relates to a method of manufacturing a non-single crystal film including an amorphous film typified by an amorphous silicon film, amorphous silicon nitride film and micro-crystal silicon film, and to a non-single crystal semiconductor device. More particularly, the present invention relates to a method of manufacturing a non-single crystal film for use in a thin film device such as a thin film transistor, an optical sensor and a solar cell, and to a non-single crystal semiconductor device.
2. Related Art
Recently, a semiconductor device using a non-single crystal semiconductor film such as a micro-crystal silicon or amorphous silicon has been energetically researched and developed. In particular, the viewpoint enabling a large area product to be realized at a low cost enhances developments of a solar cell, the photosensitive drum for a copying machine, a thin film transistor for a liquid crystal display, a solid-state image sensing device (a photosensor) for an information processing apparatus such as a facsimile machine, the weight of which is reduced. Hitherto, the non-single crystal silicon film for use in the foregoing semiconductor devices has been deposited by an RF plasma CVD method wherein silane SiH.sub.4 or disilane Si.sub.2 H.sub.6 is used as a film forming gas or a reactive sputtering method where a Si target is sputtered in Ar plasma under the presence of hydrogen gas, or an optical CVD method or a ECR-CVD method. The method for forming amorphous silicon by employing the CVD method was found by R. C. Chittic and others followed by W. E. Spear and others who have enabled the electrical conductivity of the amorphous semiconductor to be pn-controlled by using impurities. Generally, the micro-crystal silicon or the amorphous silicon obtained as above mentioned contains 10% or more hydrogen. A plasma CVD method has been most widely used among various methods for depositing the micro-crystal silicon or the amorphous silicon because a film having relatively satisfactory characteristics can be formed. The plasma CVD method is a method comprising steps of: using silane SiH.sub.4 or disilane Si2H6; performing dilution with hydrogen (H.sub.2) gas if necessary; generating plasma by using high frequency in an RF range of 13.56 MHz or in a microwave rage of 2.54 GHz; decomposing the film forming gas by utilizing the plasma to generate activator having reactivity; thus depositing on the substrate the micro-crystal silicon or the amorphous silicon. By mixing doping gas made of, for example, phosphine PH.sub.3, or diborane B.sub.2 H.sub.6 or boron fluoride BF.sub.3 at the time of forming the film, a micro-crystal silicon film or an amorphous silicon film, where the conductivity type can be determined to be n-type or p-type, the electrical conductivity and the photoconductivity are controlled, can be formed. The films, pn-controlled as described above, are also important films serving as an ohmic layer and a blocking layer of the semiconductor device. The foregoing films are used to manufacture pin-type solar cells, and photodiodes and so forth.
An a-Si film formed by a vacuum evaporation method or a sputtering method and accordingly containing no hydrogen exhibits a very high dangling bond density, for example, having a spin density of about 10.sup.20 cm.sup.-3.
Therefore, it is considered that hydrogen performs an important role improving the quality of the hydrogenated a-Si film formed by the RF plasma CVD method. In particular, when the temperature of the substrate is made to be about 250.degree. C., the spin density is lowered to about 10.sup.16 cm.sup.-3 and, therefore, the pn control by means of impurities can be performed. It has been confirmed that the a-Si formed by the RF plasma CVD method contains about 10% hydrogen resulting in an estimation that hydrogen terminates the dangling bond to improve the quality of the film. If hydrogen has the foregoing function, it could be considered that the supply of hydrogen enables the dangling bonds to be decreased even if a film is formed by the vacuum evaporation method or the sputtering method that does not use the reactive gas SiH.sub.4 containing hydrogen in the RF plasma CVD method. The foregoing fact was confirmed, resulting in the role of the hydrogen to serve as a dangling bond terminator in the a-Si film to be widely confirmed.
Furthermore, a variety of experiments have been carried out, resulting in the discovery that the process of growth of the film by the RF plasma CVD method using the SiH.sub.4 gas as the raw material is usually categorized as follows.
(1) Radical Generation Process
In this process, electrons and SiH.sub.4 molecules repeat inelastic collisions in the plasma, resulting in generations of various radicals, ions and atoms. There is a good possibility that the main precursors of the film forming reaction are SiH.sub.2 and SiH.sub.3 radicals.
(2) Radical Transportation Process
In this process, neutral radicals generated in the plasma are transported to the surface of the substrate due to diffusion while performing a variety of secondary chemical reactions mainly with the SiH.sub.4 molecules. It can be estimated that SiH.sub.3 radicals mainly reach the surface of the substrate, in view of the generation ratio of the radicals in the plasma and the reaction life in the transportation process. However, increase of the density of the radicals reaching the surface such as the Si, SiH and SiH.sub.2 will deteriorate the quality of the formed film due to the difference in the form of the reaction on the surface.
(3) Surface Reaction Process
In this process, the radicals, which have reached the surface of the grown film, are adsorbed by the surface, followed by diffusion of the surface to form chemical bonds with stable sites, resulting in amorphous network to be formed. If the temperature of the substrate is sufficiently high and the surface is covered with hydrogen, the SiH.sub.3 radicals are sufficiently diffused in the surface. As a result, the SiH.sub.3 radicals are chemically bonded with stable sites, resulting in a film exhibiting high quality to be obtained.
As a result of the foregoing film forming mechanism, the SiH.sub.3 radicals serving as the precursors of the deposition reaction of the a-Si film are selectively diffused in the surface of the substrate. The diffusion of the surface of the substrate enables an a-Si film exhibiting high quality to be formed. It is considered important that the surface of the substrate is covered with hydrogen in order to enhance the surface mobility of the radicals. Furthermore, it is considered that the surface reaction is carried out in the following manner that: the SiH.sub.3 radicals extract the surface hydrogen in the surface covered with hydrogen followed by chemical reactions between the formed site and the other SiH.sub.3 radicals.
Depending upon the foregoing theory, manufacturing conditions such as the film forming temperature, the quantity of the flow of the raw material gas, the pressure and the electric power to be applied have been improved in order to improve the quantity of the a-Si film. However, it has been confirmed that an optimum temperature for forming the film exists and the increase in speed at which the film is formed deteriorates the quantity of the film. That is, there is a film forming temperature at which the spin density becomes minimum as shown in FIG. 1.
If the film forming speed is raised, increase in hydrogen in the film and in the dangling bond density, which deteriorate the characteristics of the thin a-Si film also occurs. For example, the photoconductivity .sigma..sub.p (S/cm), which is one of the basic characteristics of the thin film, deteriorates as shown in FIG. 2.
As a result, the film forming conditions such as the flow rate of the raw material gas, the pressure and the applied electric power to be employed in the foregoing example method of manufacturing the a-Si thin film which are capable of maintaining the characteristics of the device are as follows: the temperature is about 250.degree. C.; and the conditions realizing the film forming speed of about 0.2 to 2 .ANG./sec. The a-Si film thus formed has a spin density of about 10.sup.16 to 10.sup.17 and content of hydrogen is about 10%. The foregoing physical properties are not considered to be the optimum values. If hydrogen is required to terminate the dangling bond, it can be considered that it is a necessity to contain about 1% hydrogen.
Although hydrogen contained in the film performs an important role to compensate the dangling bond, there arise the following problems due to the concentration of hydrogen.
The film formed by using the RF wave contains hydrogen by 10% or more, which is considered to be the cause of deterioration by light. Further, damage of the film due to ions in the plasma is critical and, accordingly, the density of defects in the film cannot be lowered than about 10.sup.15 /cm.sup.3. Accordingly, an attempt has been made to prevent the light deterioration by reducing hydrogen in the film. For example, a method has been suggested in which a hydrogen plasma process and film forming are repeated to reduce hydrogen contained in the film, thus resulting in prevention of the light deterioration (31a-ZD-11, spring 1990, or 28-P-MD-1, autumn 1990 of Lecture Meeting of Concerned Associations of Applied Physics Society). However, the foregoing method encounters a problem in view of a practical point that the apparatus cannot easily be constituted, resulting in a difficulty in mass production to arise. On the other hand, an attempt has been made in which the conventional RF is used to maintain the temperature of the substrate at 350.degree. C. followed by raising the film forming speed in the foregoing state, resulting in reduction in the density of defects in the film (30p-ZT-3,4, spring 1992, Lecture Meeting of Concerned Associations of Applied Physics Society). However, the foregoing method encounters a problem due to the use of the RF that the pressure is raised and the flow rate ratio is changed, resulting in very severe conditions for the film forming plasma. Therefore, abnormal discharges take place and reactions can easily be occur in a gas phase, causing the polymer to be formed followed by received into the film while being formed into particles. As a result, the quality of the film easily deteriorates, resulting in unsolved problems in terms of the reproductivity and mass-production facility.
Accordingly, there have been suggested a variety of methods for improving the quality of the a-Si film by using the foregoing RF plasma CVD method as the base.
A chemical annealing method has been disclosed in p. 1618, Vol. 59 (1990), Applied Physics, reported by a group including Shimizu, Tokyo Institute of Technology. According to the report, an assumption is made that the light deterioration of the a-Si film and the like are caused from the non-uniformity of the network structure of Si, and, therefore, the network structure of Si is finely constituted to stabilize the structure. The foregoing object is achieved by controlling the process of forming the structure in which elimination of hydrogen is accompanied in the surface in which the film grows by supplying hydrogen in the form of atoms which have strong chemical effect mutual with Si. The hydrogen in the form of atoms are generated by a large quantity by microwave plasma followed by conveyance to the deposition portion. A usual RF grow discharge decomposes SiH.sub.4, resulting in decomposition of the substrate. Accordingly, the time (t.sub.1 second), in which SiH.sub.4 is supplied, is controlled, and the film deposition and the process (T.sub.2 second) with hydrogen in the form of atoms on the surface of the deposited film are repeated. By repeating the foregoing deposition surface treatment, the content of hydrogen in the a-SI film can be reduced to about 1%. Thus, resulted improvement in the mobility of the carrier and the prevention of the light deterioration have been confirmed.
As a method of modifying the foregoing RF plasma CVD method, an example has been disclosed in which a raised frequency is used in the high-frequency discharge in the RF band range.
That is, a group including Oda, Tokyo Institute of Technology, has disclosed a method in Lecture Meeting of Concerned Associations of Applied Physics Society, autumn 1990, and spring in 1991 (28p-MF-14 and 28p-S-4) where the discharge is performed at a high frequency of 144 MHz, resulting in amorphous silicon to be manufactured followed by evaluation.
However, a simple examination is made at frequencies of 13.56 MHz and 144 MHz and the optimum frequency in the VHF band for enlarging the area and improving the productivity has not been found.
Another disclosure has been disclosed, in Japanese Patent Laid-Open No. 3-64466, which utilizes the effect of the frequency and in which a raised frequency is used, resulting in a spatially uniform discharge and a uniform film forming speed to be realized. However, the foregoing invention has simply discussed the uniform film forming, and no description was made about the influence and the technological effect of the high frequency in the VHF range upon the quality of the film.
In Japanese Patent Laid-Open No. 2-225674, a method using a frequency ranged from 1 kHz to 100 MHz has been described, but no description has been made about the technological operation and effect in the VHF band, resulting in only a technological expansion in the RF band.
In U.S. Pat. No. 4,933,203, high frequency waves in the VHF band were used followed by evaluations of the formed films, resulting in the optimum relationship between the frequency and the distance between electrodes. However, the foregoing relationship is insufficient for problems to be described later as unsolved problems.
Although a variety of disclosures have been made about VHF, large number experiments have been carried out simply by raising the frequency to the VHF band.
Then, the problems will be described more specifically.
With the technological development made recently, there arises a desire of improving the quality of the a-Si thin film in a variety of fields, the solar cell, the liquid crystal TV, and photosensor. However, the conventional a-Si thin film formed by utilizing the RF discharge at 13.56 MHz encounters the following unsolved problems in viewpoint of an application to an a-Si thin film device.
(1) Problems in a viewpoint of the basic characteristics of the thin film:
The attained carrier mobility is insufficient when it is adapted to the thin film transistor. The S/N ratio defined by the photoconductivity and the dark conductivity ratio is too small when it is adapted to the photosensor. A critical light deterioration takes place, in which the characteristics of the photoconductivity (.sigma..sub.p) due to irradiation with light, when it is adapted to a solar cell.
(2) Problem in a viewpoint of the manufacturing yield When it is employed in a large-area device, the distribution of the characteristics of the device, realized due to the distribution of the film thicknesses and the qualities of the films, causes the yield to deteriorate.
(3) Problem in a viewpoint of the cost
Since a high grade film adaptable to a thin film device can be formed only at a low film forming speed, the manufacturing performance cannot be improved, resulting in a difficulty in reducing the cost.
That is, in order to improve the characteristics of a large-area a-Si thin film device, to improve the yield and to reduce the cost, the film must be formed at a high speed while improving the basic characteristics of the a-Si thin film.
In order to achieve this, various attempts have been made to improve the manufacturing conditions for the plasma CVD method at 13.56 MHz, such as the flow rate of the raw material gas, the pressure, and the electric power to be applied. However, the rise of the film forming speed increases hydrogen in the film, which is assumed to deteriorate the characteristics of the a-Si thin film, and generation of foreign matters (polysilane), which deteriorate the yield. For example, the increase in the film forming speed deteriorates the photoconductivity .sigma..sub.p (S/cm) which is one of the basic characteristics of a thin film. As described above, a film capable of improving the characteristics of a thin film transistor type photosensor to be formed into a device has not been manufactured as yet. As a result, the foregoing method of manufacturing the a-Si thin film allows a film forming speed of about 0.2 to 2 .ANG./sec as the speed that is capable of maintaining the characteristics of the device.
Although the RF discharge at 13.56 MHz exhibits an advantage that a film can be formed over a wide area, it encounters a problem that the film forming speed is too low and the substrate, that is, the thin film is critically damaged by ions. The microwave discharge at 2.54 GHz exhibits a high film forming speed and capable of protecting the substrate from damage due to ions. However, it encounters a problem that a large area film cannot easily be formed. Further, the light CVD method suffers from a problem of the quality of the formed a-Si thin film and a problem of provability of forming a film over a large area. Therefore, it can be said that the light CVD method is a method that is developing. Similarly, the ECR-CVD method is able to control freely the damage of the substrate to improve the quality of the a-Si thin film, while encountering a problem that the film cannot be formed over a large area.
The aforesaid conventional manufacturing methods respectively encounter the following problems when microcrystal silicon containing impurities is manufactured. The following method suffers from unsatisfactory efficiency of utilizing the gas, the method being arranged in such a manner that silane gas is used, dilution with hydrogen gas is performed if necessary, plasma is generated at a high frequency of 13.56 MHz in the RF band range to decompose the film forming gas by the generated plasma, and microcrystal silicon, is deposited on the substrate. Therefore, a problem of unsatisfactory doping efficiency arises when the microcrystal silicon containing impurities is manufactured by introducing the impurity gas. Even more detrimental, the conventional method must provide severe condition when microcrystallization is caused to occur in the film, and a desired microcrystal film cannot be easily formed. Therefore, it is difficult to improve the doping efficiency. Then, a case where n-type microcrystal silicon film is formed will be considered. It was formed under standard conditions: silane gas was used by 3 sccm, phosphine gas of 150 sccm diluted by hydrogen gas to 100 ppm was used, the pressure was 0.5 Torr, and the RF power was 50 mW/cm.sup.2, resulting in a doping efficiency of 10%. It can be said that 90% of phosphorus in the film does not serve as the dopant.
An amorphous silicon nitride film has been used to serve as a gate insulating film for a thin film transistor or to serve as a passivation film of the foregoing device. Hitherto, the amorphous silicon nitride film for use in the forgoing semiconductor devices has been deposited by the RF plasma CVD method in which a gas formed by mixing silane SiH.sub.4 or silane fluoride SiF.sub.4 with ammonia NH.sub.3 or nitrogen N.sub.2 is used as the film forming gas. Although the silicon nitride can be manufactured by another method, that is, heat CVD method, the heat CVD method must set the temperature, at which the amorphous silicon nitride is allowed to grow, to a high temperature of about 850.degree. C. Therefore, the foregoing method cannot be employed when the aforesaid device is manufactured by using the amorphous silicon semiconductor having a low heat resistance of about 400.degree. C. However, the RF plasma CVD method is allowed to set the growth temperature to about 300.degree. C., resulting in that it can be employed. Since the decomposition and the reaction take place easier when ammonia is used than a case where nitrogen is used, a plasma CVD method is usually employed. The plasma CVD method is a method in which a mixture gas of silane SiH.sub.4 and ammonia NH.sub.3 is used, dilution with hydrogen is performed if necessary, and plasma is generated at a high frequency of 13.56 MHz, and the film forming gas is decomposed by the plasma to generate an activator having reactivity so as to deposit the amorphous silicon nitride film on the substrate. However, the ammonia gas is a corrosive gas, resulting in a difficulty in handling. Therefore, there sometimes occur a problem for the manufacturing apparatus. On the contrary, the nitride gas exhibits an advantage in the handling facility. In addition, the nitride gas can be purified easier than the ammonia gas and, therefore, the entry of the impurities into the formed film can be reduced. The impurities in the film must be reduced because they exert bad influence upon the electric characteristics of the amorphous silicon nitride insulating film. It has been that, when the film is formed by using the ammonia gas, the content of hydrogen in the film increases in comparison to that when the same is formed by using the nitrogen gas. If hydrogen in the film increases, the density lowers, resulting in deteriorations in the precision and heat resistance. What is worse, hydrogen is diffused in the film, causing a variety of unstable phenomena to occur. Therefore, hydrogen must be reduced in the film. As described above, a variety of advantages can be attained when the mixture gas of the silane gas and the nitrogen gas is used. In this viewpoint, it is more advantageous when a mixture gas of silane fluoride gas and nitrogen gas containing no hydrogen is used.
Since the composition of the amorphous silicon nitride thin film is varied depending upon the manufacturing method and condition, it is usually expressed as SiNx film. The physical properties of the SiNx film are considerably changed depending upon the composition and the content of hydrogen. The methods of depositing the SiNx film are typified by a reduced-pressure or normal pressure CVD method by using the foregoing SiH.sub.4 --NH.sub.3 mixture gas, a plasma CVD method and a light CVD method. The RF plasma CVD method, in which the discharge frequency of 13.56 MHz of the foregoing SiH.sub.4 --NH.sub.3 mixture gas is utilized, has been most widely employed because the reaction can easily occur and satisfactory controllability can be attained.
However, the foregoing SiNx film sometimes encounters the following problems: when the SiNx film is used as the gate insulating film of a thin film transistor (hereinafter abbreviated to "TFT") using an a-Si thin film, the characteristics of the a-Si TFT change the device characteristics depending upon the SiNx gate insulating film. If the threshold voltage is changed, the ON-OFF ratio is lowered or the responsibility deteriorates, the yield deteriorates, resulting in a critical problem to occur. Also a TFT type photosensor encounters changes in the photoelectric current and the dark current which are the basic characteristics thereof due to the change in the threshold voltage. The foregoing problems occurring in the device characteristic are usually considered to be causes from the characteristics of gate insulating film or from the interface between the gate insulating film and the a-Si film.
Specifically, it has been considered that the change in the threshold is caused from injection of electrons or positive holes from a-Si to SiNx or capture of the same by the trap level of SiNx. Therefore, it has been considered that a high quality film having a large optical band gap of the SiNx film and having a small electronic spin density is effective to minimize the change.
In terms of the content of hydrogen in the foregoing film, if the content of hydrogen in the SiNx film is large, the density of the film is lowered, resulting in that the voltage resistance to deteriorate. The foregoing hydrogen is diffused in the film, causing a variety of unstable phenomena to occur. Therefore, it is preferable that hydrogen in the film is reduced.
Another assumption is made that the characteristics of the TFT can be improved by shifting the stress of the SiNx film toward somewhat the compression side.
Therefore, the basic physical properties of the SiNx thin film have been investigated from a variety of viewpoints such as the film forming conditions and the experiment conditions, resulting in that a SiNx thin film having a large optical band gap, a small spin density, a low hydrogen content and the stress positioned somewhat adjacent to the compression side will realize a satisfactory result.
There arises the following problem experienced with the aforesaid conventional method in which the mixture gas of the silane gas or the silane fluoride and the nitrogen gas is used, dilution with the hydrogen gas is performed if necessary, plasma is generated at a high frequency of 13.56 MHz, the film forming gas is decomposed by the plasma, and thereby the amorphous silicon nitride film is deposited on the substrate:
Since the nitrogen gas cannot be easily decomposed as compared with the ammonia gas, a large high-frequency electric power must be supplied. However, the raising of the supplied electric power will enlarge the quantity of gas removed from the wall of the chamber, resulting in an enlargement of impurities introduced into the film. Furthermore, damage due to the plasma becomes critical, thus deteriorating the characteristics of the film. Further, silicon fluoride is more chemically stable than silane, resulting in a low decomposition efficiency in the plasma. Therefore, if it is used, the efficiency of using the gas is unsatisfactorily deteriorates. What is worse, the film forming speed is lowered undesirably.
Although a variety of attempts have been made with the conventional RF plasma CVD method using the frequency of 13.56 MHz for the purpose of meeting the foregoing physical properties required for the thin film, the following problems occur:
The hydrogen content C.sub.H (%) in the film shows the highest dependency upon the substrate temperature T.sub.S, and it also depends upon the type of the raw material, resulting in a relationship as shown in FIG. 3. As can be understood from FIG. 3, the density of hydrogen can simply be lowered by changing the raw material gas from NH3 to N2 and by raising the temperature. However, there is an upper limit of about 400.degree. C. in terms of use of a large-size glass, the structure and the manufacturing facility. Furthermore, there is a lower limit of about 250.degree. C. in terms of the device characteristics. The reason for this is that the thin film has a high spin density if the temperature is low level, thus deteriorating the reliability of the device characteristics. That is, the range designated with black dots is considered to be the range with which the device characteristics and the manufacturing facility can be maintained at the satisfactory level.
The relationship between stress (dyn/cm.sup.2) and the hydrogen content C.sub.H (%) is shown in FIG. 4. In proportion to reduction in the content of hydrogen, the stress is shifted from tensile stress to compression stress. However, the stress of a high quality SiNx film obtainable from the foregoing temperature range from 250.degree. C. to 400.degree. C. or lower and using the NH.sub.3 is mainly tensile stress. If N.sub.2 is used, a large compression stress is attained. The black dots in FIG. 4 are points at which the high quality SiNx thin film can be realized, while the diagonal-line regions designate controllable hydrogen contents and stress. That is, it can be understood that the SiNx thin film having the somewhat compression stress, that is, a compression stress of 5.times.10.sup.9 dyn/cm.sup.2, at which satisfactory device quality can be attained, cannot be manufactured.
If NH.sub.3 is used, the content of hydrogen in the film can be controlled by changing the ratio of NH.sub.3 and SiH.sub.4, as shown in FIG. 5. As can be understood from FIG. 5, lowering of the content of hydrogen lowers the ratio N/Si, resulting in that the optical band gap is narrowed and the quality of the film deteriorates. Therefore, the ratio of NH.sub.3 and SiH.sub.4 cannot be lowered. Similarly, the control of the stress by enlarging the content of hydrogen deteriorates the quality of the film such as the ratio N/Si in the case where N.sub.2 is used.
That is, there is a problem in that the film forming conditions cannot be obtained, with which the stress can be somewhat shifted to the compression side, the ratio N/Si at this time can be made to be in the neighborhood of the stoichiometric ratio, the optical band gap can be enlarged, and the spin density can be lowered.